The purpose of this continuing project is to better understand the roles of altered hematopoietic stem or progenitor cell (HPC) differentiation and cytogenetic damage in early events in the development of acute myelogenous leukemia (sAML) following treatment with alkylating chemotherapeutic agents or benzene. Consistent with emerging models of carcinogenesis, sAML is an evolutionary process involving selection of clonal cytogenetic events in HPC together with microenvironmental changes in stromal cells that ultimately contribute to the development of myelodysplasia (MDS) and sAML. During the previous funding period a multistep model for the evolution of sAML was developed in which drug- induced alterations in HPC proliferation lead to development of a distinct pattern of clonal cytogenetic abnormalities (C.A.) and progressive dysplastic changes that select for the emergence of the malignant cell phenotype. Using this model as a hypothesis, the pattern of expression of genes known to be involved in regulation of normal hematopoiesis will be compared between normal human bone marrow (BM) cells and BM treated with alkylating agents in vitro. Treatment related changes in transcriptional regulation of gene expression occurring in purified sub-populations of human CD34+ BM cells in liquid culture will be assessed using a variety of molecular biology techniques and correlated with flow cytometric analysis of cell phenotype, clonogenic function in HPC, and cytokine production in stromal cells. Fluorescence in situ hybridization also will be used to evaluate the potential of individual agents to produce C.A. in specific sub-populations of CD34+ human BM cells. Finally, patterns of gene expression will be correlated with genotype, phenotype and functional clonogenic response in BM cells obtained from patients with MDS or evolving sAML.